Ammonium and nitrate are primary nitrogen sources for plant growth and development. Plants require transporters for acquisition of both ammonium and nitrate. Transporters of ammonium and nitrate exist not only in plants, but in almost all organisms. Ammonium transporters (AMTs) usually exist in a genome as gene families, for example at least: six in Arabidopsis thaliana, eight in Chlamydomonas reinhardtii (Gonzales-Ballester et al. (2004) Plant Molec Biol 56: 863-878), fourteen in poplar (Couturier et al. (2007) New Phytologist 174: 137-150), six in diatom Phaeoactylum tricornutum (Allen (2005) J Phycology 41).
Based on phylogenetic analysis, three subfamilies of ammonium transporters were identified (Loqué & von Wiren (2004) J Exp Bot 55(401): 1293-1305):                1. the AMT subfamily, including the plant AMT1-type transporters, and cyanobacterial ammonium transporters;        2. the MEP subfamily, including the plant AMT2-type transporters, the yeast MEP transporters, the E. coli AmtB, and other prokaryotic homologues;        3. The Rh subfamily, including only human and animal Rhesus blood group antigens.        
All AMT polypeptides are highly hydrophobic membrane proteins with at least 10, more commonly 11, putative transmembrane spanning helices. The AMT polypeptides have been shown in numerous reports to be able to uptake ammonium over a wide concentration range, although with different affinities from organism to organism. Within certain organisms, such as plants, high and low affinity ammonium transporters were identified (Gazzarini et al. (1999) Plant Cell 11:937-47). In addition to affinity properties, several other regulatory mechanisms have been identified for ammonium uptake, for example at transcriptional and post-transcriptional levels (Yuan et al. (2007) Plant Phys 143: 732-744).
Over-expression of a nucleic acid sequence from rice encoding an AMT1 was performed in two rice cultivars (Taipei 309 and Jarrah), using a maize ubiquitin promoter for constitutive expression. Shoot and root biomass of transgenic lines decreased during seedling and early vegetative stage compared to wild type, especially when grown under high ammonium nutrition (Hogue et al. (2006) Functional Plant Biol 33: 153-163). The authors concluded that decreased biomass of the transgenic plants at early stages of growth might have been caused by the accumulation of ammonium in the roots owing to the inability of ammonium assimilation to match the greater ammonium uptake.
In U.S. Pat. No. 6,620,610, is described a nucleic acid sequence encoding an AMT1 polypeptide from Arabidopsis thaliana, plasmids comprising the nucleic acid sequence encoding an AMT1 for expression in yeast and bacteria.
In U.S. Pat. No. 6,833,492 are described nucleic acid sequences encoding an AMT1 polypeptide from soybean, corn, wheat, and rice. A nucleic acid sequence encoding an AMT1 polypeptide or an AMT polypeptide having 90% amino acid sequence identity to the isolated soybean AMT1 polypeptide is described. Plants and seeds comprising a recombinant nucleic acid sequence encoding such a polypeptide sequence are described, as well as methods to produce such plants.
Surprisingly, it has now been found that increasing expression of a nucleic acid sequence encoding an AMT polypeptide gives plants having increased yield-related traits relative to control plants.
According to one embodiment, there is provided a method for increasing yield-related traits in plants relative to control plants, comprising increasing expression of a nucleic acid sequence encoding an AMT polypeptide as defined herein, in a plant. The increased yield-related traits comprise one or more of: increased early vigour, increased aboveground biomass, increased root biomass, increased total seed yield per plant, increased seed filling rate, increased number of filled seeds, increased number of flowers per panicle, and increased harvest index.